Thermally sensitive thyristors are known which intrinsically switch from a nonconductive off state to a conductive on state in response to temperature. Examples of such thyristors are disclosed in Jaskolski et al U.S. Pat. No. 3,971,056, issued July 20, 1976, and Jaskolski et al U.S. Pat. No. 4,112,458, issued Sept. 5, 1978.
The breakover voltage of these thyristors varies with temperature as shown in FIG. 1 which plots temperature on the horizontal axis and breakover voltage (voltage between the anode and cathode of the thyristor) on the vertical axis. The curve may be determined empirically or may be calculated from the design parameters of electrical base widths, minority carrier lifetimes and relative layer resistivities.
In certain applications it may be desirable to use this known relationship between breakover voltage and temperature to determine the value of one as a function of a sensed or measured value of the other. A steep slope provides extreme accuracy and high resolution of temperature when read by voltage variation. However, a steep slope only affords a narrow range, and in certain applications a wider temperature sensitive region may be desirable or necessary.
It is known that the value of resistance 7, FIG. 2 between the gate and cathode of the thyristor affects the switching temperature (the narrow temperature region in which the breakover voltage drops sharply, FIG. 1). As the resistance decreases, the switching temperature increases, FIG. 3. This known effect has been used in prior thyristors to shift the critical region left or right in FIG. 1. The critical region does not expand, but rather shifts, with the width of the region remaining the same.
A circuit employing a thyristor in combination with a field effect transistor is disclosed in Clark, U.S. Pat. No. 3,812,405, issued May 21, 1974. The transistor used there, however, is a very ordinary field effect transistor, as indicated by the geometric drawings of that patent. Thus the source to drain resistance is in the area of 100 ohms. The result is that again the critical region is not widened, but rather is merely shifted to the left or right.
The present invention deals with widening the region of temperature sensitivity.